Humans show a wide range of individual differences in susceptibility to drug addiction. An understanding of the basis for these differences, which can include both risk and protective factors, may help to understand the etiology of substance abuse and dependence (SAD). This proposal aims to study individual differences in mRNA editing of the serotonin 2C receptor (5-HT2CR) as a potential vulnerability factor for SAD. RNA editing is a post-transcriptional mechanism that can alter one or more translational codons, thereby giving rise to several functionally distinct proteins from a single gene. mRNA editing of the 5-HT2CR produces numerous isoforms, each of which has its unique signaling properties, with more highly edited isoforms exhibiting decreased function. We recently found increased 5-HT2CR editing in the dorsolateral prefrontal cortex (DLPFC) of suicide victims and in the medial prefrontal cortex (mPFC) of rats with high propensity to self-administer addictive drugs. In addition, several lines of evidence indicate an association among addictive behaviors, impulsivity and suicide. This association has been hypothesized to stem from common genetic or epigenetic contributions that result in impaired prefrontal inhibitory modulation. Thus, we hypothesize that regulation of 5-HT2CR function via editing in the relevant brain circuits may influence inhibitory control over addictive and impulsive behaviors. Individual differences in 5-HT2CR editing in these circuits may, therefore, contribute to propensity toward SAD. To test this hypothesis, we will employ two independent, but interrelated, approaches. First, we will use postmortem specimens from three relevant brain regions [DLPFC, orbitofrontal cortex, and nucleus accumbens (NAcc)], to determine if 5-HT2CR RNA editing differs between humans who abused heroin and those who did not. In addition, we will examine whether or not heroin intake induces changes in 5-HT2CR editing. This will include addressing the possibility that the effect of heroin on editing, if it exists, may differ depending on inherent liability to drug abuse. To this end, we will employ an animal model to compare two phenotypes of rats that represent the extremes in the propensity to self-administer a variety of addictive drugs. The results will also provide important insights in interpreting the human data in which innate vs. heroin-induced differences in drug addicts may be difficult to dissociate. In both human and rat studies, 5-HT2CR editing will be analyzed using Next Generation Sequencing, which provides substantially increased precision and sensitivity compared to previously used methods. Since RNA editing has never been examined in relation to drug abuse vulnerability in humans, the proposed study constitutes a high-risk but high reward undertaking. If our hypothesis is supported, this research holds promise to provide important insights into the biological underpinnings of drug addiction vulnerability as well as to identify novel approaches for the development of treatment strategies for SAD. PUBLIC HEALTH RELEVANCE: Humans and animals show a wide range of individual differences in susceptibility to drug addiction. The biological underpinnings of addiction, however, remain elusive. This proposal aims to study mRNA editing (a recently discovered epigenetic mechanism) of the serotonin 2C receptor as a potential vulnerability factor for substance abuse and dependence. The proposed research has the potential to provide important insight into the etiology of addiction, as well as to identify novel targets (related to the RNA editing process) for the development of more effective treatment strategies.